Conventionally, there is known in the art a developer cartridge including a detection protrusion that can be in contact with an actuator provided at a housing of an image-forming apparatus, and a detection gear including the detection protrusion (for example, refer to Japanese Patent Publication No. 4848632). Specifically, in this art, the detection protrusion pushes the actuator when the developer cartridge is attached to the image-forming apparatus, and when a drive force is input to the cartridge thereafter, rotation of the detection gear causes the detection protrusion to push the actuator further and then to move away from the actuator. Further, in this art, the number of detection protrusions varies according to the specification of the developer cartridge. With this structure, how many times the actuator is pushed by the detection protrusion(s) is configured to be detected by a control device, thereby allowing the control device to determine the specification of the developer cartridge.